This application relates to an adhesive for joining surfaces formed of various materials including polymers, paper, metals and ceramics, particularly but not exclusively, polypropylene. The invention finds particular but not exclusive application in high-speed manufacture of packaging.
Bonding of polypropylene sheets to form boxes or containers using conventional packaging equipment has been limited by the high speeds, e.g. 180 m/min at which the sheet materials need to be handled for commercial viability. Conventional polypropylene adhesives cannot be applied at such speeds whilst retaining good bonding properties.
According to a first aspect of the present invention an adhesive composition comprises:
4) and optional further ingredients selected from stabilisers, fillers and waxes;
wherein the proportions of 1, 2 and 3 are selected to total 100%.
According to a second aspect of the present invention, a method of forming a polypropylene article comprises the steps of:
applying a quantity of an adhesive in accordance with the first aspect of this invention to a first surface of a workpiece,
contacting a second surface of the workpiece to the adhesive; and
allowing the adhesive to set to form said article,
wherein tie first and second surfaces are secured together by the adhesive.
The article is preferably polypropylene packaging and the workpiece is a blank for manufacture of the packaging. The packaging may be completely composed of polypropylene or may comprise polypropylene in addition to one or more different materials.
An adhesive in accordance with the present invention may confer several advantages. The adhesive has good adhesion properties and can be applied at high speed without tacking or stringing to polypropylene or other sheet materials using conventional packaging equipment which may process up to 15,000 articles per hour or more providing the folding and dispense technology has the capacity to perform at higher rates.
Compositions in accordance with the present invention not only can be used for self-bonding of polypropylene but may be used for a wide range of different substrates including other polymers paper, ceramics and metals. The compositions can be readily formulated to accommodate rheological and open-time demands of a wide range of machinery used for manufacture of different types of packaging, including composite products when polypropylene is present together with another polymer, paper, ceramic or metal.
When used to bond polypropylene the composition has the advantage that the adhesive, also being composed predominantly of polypropylene does not introduce additional polymers into the product, facilitating recycling of the bonded composite.
Compositions in accordance with the invention are transparent and preferably non-coloured, allowing use on transparent sheet materials. Preferred polypropylenes used in accordance with this invention have good intrinsic thermal and oxidative stability or can be made more stable with small amounts of conventional additives.
Percentages and proportions used in this specification are by weight unless indicated otherwise.
Preferably about 70% of the amorphous polyalphaolefin is used. Preferred polyalphaolefins include ethene/propene/1-butene copolymers. Atactic polypropylene is preferred. A particularly preferred polypropylene is Vestoplast 704 (manufactured by Dagussa-Hxc3xcls AG).
Preferred amorphous polyalphaolefins have a melt viscosity at 190xc2x0 C. of 2,700-50,000, preferably 3,000-4,000 mPa.s (cp).
Various tackifiers may be employed, including hydrocarbon resins, rosin esters and other resins of natural origin and hydrogenated resins. Preferred tackifiers have low viscosity, clarity and thermal stability. C5 hydrocarbon oligomer resins with Mw 500 to 1,500, preferably 700 may be employed. The preferred resins include Regalite resins manufactured by Hercules, for example, Regalite R1090.
The functionalised polypropylene may be produced directly by reactive modification by using radical chemistry where the functional group can be isocyanate, anhydride, amine, alcohol or acid. The functionalised polypropylene may also be made by copolymerisation of propylene with a suitable diolefinic monomer and then subsequent modification of the pendant unsaturation. Maleic anhydride grafted polypropylenes may be employed. Alternatively 2,2xe2x80x2-dimethyl-1,3-isopropenyl benzyl isocyanate (also known as dimethyl meta-isopropenyl benzyl isocyanate (TMI)) modified polypropylenes as disclosed in WO96/34024, WO96/34031 and WO98/13398 may be employed.
The functionalised polypropylenes may be used to give cohesive strength and adjust the rheological behaviour to increase the adhesion properties. Use of functionalised polypropylenes has the advantage that the adhesive may be recycled together with polypropylene packaging to which it has been applied.
Further ingredients may include UV stabilisers and fillers. Hydrocarbon waxes may be used to modify the viscosity if necessary. However, use of hydrocarbon waxes is not preferred.
The invention is further described by means of example but not in any limitative sense.